AB0724 THE VALUE OF [18F]FDG PET/CT FOR MONITORING AND TREATMENT DECISION GUIDANCE IN LARGE VESSELL VASCULITIS: IS IT TELLING TOO LITTLE, OR MAYBE TOO MUCH?
Authors: M. Di Cicco, O. M. Epis, E. Verduci, C. Casu, G. Segatto, F. Filippini, C. Rossetti, D. A. Filippini
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> Large Vessel Vasculitis (LVV) are vasculitides affecting large arteries, including aorta and its major branches. LVV can cause systemic and vascular inflammation that could result in long term complications such as artery stenosis, aneurysm formation and artery dissection. LVV include giant cell arteritis (GCA) and Takayasu. CGA typycally affects people aged over 50 and involves superficial temporal arteries. Meanwhile its cranial manifestations are well recognized and can be easily diagnosed by vascular ultrasound, it has been appreciated that GCA can extend beyond the superficial cranial arteries in up to 80% of patients or even spare the temporale arteries, and ultrasound is of limited diagnostic value in these cases. The usefulness of [<sup>18</sup>F]FDG PET/CT in supporting the diagnosis of LVV is well known, particularly in patients with less specific manifestations such as fever or unexplained weight loss. However, the role of [<sup>18</sup>F]FDG PET/CT in monitoring the course of the disease and its complications is less well established. <h3>Objectives</h3> To investigate the findings of repeated [<sup>18</sup>F]FDG PET/CT scan in a cohort of patients with LVV in clinical and biochemical remission. <h3>Methods</h3> We have evaluated 30 pts (female 67%, age 75±9) with a diagnosis of LVV based on clinical features and serological tests (T0). The diagnosis was also supported by a positive [<sup>18</sup>F]FDG PET/CT scan (arterial uptake equal or superior to the liver at the aorta or at least one of its major branches). 16 patients (53%) had also GCA confirmed by positive ultrasound of the temporal arteries. All patients were started on Prednisolone (PRED) 1 mg/kg according to standard guidelines, and some were subsequently started on Tocilizumab (TCZ) due to either relapse on steroid tapering or contraindications to long term use of steroids. After at least 6 months of effective treatment and while on clinical and biological controlled disease (T1), patients were reassessed clinically and biochemically; 14 pts aldo had a repeated [<sup>18</sup>F]FDG PET/CT scan. We have used a binary grading to score the second scan: 1. negative (improved compared to first study) or 2. positive (not improved compared to first study), where improvement has been defined as no uptake or uptake inferior to the liver in the same areas that were highly metabolically active at previous study, and no new sites of vascular uptake detected. Images have been assessed by a nuclear medicine doctor and reviewed by 2 rheumatologists with recognised experience in the field of vasculitides, with excellent interobserver agreement. <h3>Results</h3> Inflammatory markers significantly dropped between the 2 timepoints (ESR 48±28 vs 6±4 and CRP 5.9±5.4 vs 0.4±1.2, p=0.0004). Among the 14 patients that had a repeated scan, 9 (65%) showed a negative and 5 (35%) a still positive scan despite being in clinical and biochemical remission. The number of metabolically active sites was reduced with a statistically significant difference between the 2 groups (8±4 vs 3±3, p=0.02). Of the 5 pts that had a positive scan, only 1 (20%) had a positive ultrasound of the temporal arteries at the time of the diagnosis. <h3>Conclusion</h3> Our study revealed persistent vascular uptake on repeated [<sup>18</sup>F]FDG PET/CT scan in over 1/3 of LVV patients with clinical and biological controlled disease. The significance of this residual uptake and its prognostic value is currently unknown. Therefore the routine use of [<sup>18</sup>F]FDG PET/CT alone in monitoring disease activity and treatment response may be not appropriate and therapy should not be modified based on scan results only. Further research is needed in order to place the correct use of imaging in the long term monitoring of LVV. <h3>References</h3> [1]Dejaco C, et al. Ann Rheum Dis 2018;77:636–643. doi:10.1136/annrheumdis-2017-212649 [2]Duftner C, et al. RMD Open 2018;4:e000612. doi:10.1136/rmdopen-2017-000612 [3]van der Geest KSM et al. Eur J Nucl Med Mol Imaging 2021; 48:3886–3902 <h3>Acknowledgements:</h3> NIL. <h3>Disclosure of Interests</h3> None Declared.